Electric heating – Metal heating – By arc
Reexamination Certificate
2000-11-21
2002-04-23
Shaw, Clifford C. (Department: 1725)
Electric heating
Metal heating
By arc
C219S130210, C219S130510
Reexamination Certificate
active
06376802
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention generally relates to the art of AC pulsed arc welding utilizing a consumable electrode and, more particularly to a method of controlling the consumable electrode AC pulsed arc welding and a welding power supply apparatus therefor, which are effective to avoid an arc interruption, which is apt to occur upon re-ignition subsequent to polarity switching of the electrode, thereby to provide a high weld quality in a workpiece to be welded.
2. Description of the Prior Art
The AC pulsed arc welding is well known in the art and is a process wherein, while a welding wire is supplied at a predetermined feed speed, a first arc generating interval with the welding wire held at a positive polarity (which interval is hereinafter referred to as an “EP (electrode-positive) arcing interval”) wherein an arc is generated between the welding wire and the workpiece to be welded that are then polarized to positive and negative poles, respectively, and a second arc generating interval with the welding wire held at a negative polarity (which interval is hereinafter referred to as an “EN (electrode-negative) arcing interval”) wherein an arc is generated between the welding wire and the workpiece to be welded that are then polarized to negative and positive poles, respectively, are alternately repeated. Accordingly, the welding current flows from the welding wire towards the workpiece to be welded during the EP arcing interval and from the workpiece to be welded towards the welding wire during the EN arcing interval.
The AC pulsed arc welding process is largely employed for welding aluminum, aluminum alloy, stainless steel, steel or the like and is often employed particularly where the workpiece to be welded is in the form of a thin plate of any of those metallic materials having a few millimeter or less in thickness. The reason for frequent use of the AC pulsed arc welding in welding the thin plate will now be discussed.
In the practice of the AC pulsed arc welding, the workpiece to be welded assumes a negative pole during the EP arcing interval and the heat input to the workpiece increases as a result of a voltage drop at the negative pole. On the other hand, during the workpiece to be welded assumes a positive pole during the EN arcing interval and the heat input to the workpiece occurs as a result of a voltage drop at the positive pole, but the amount of voltage dropped at the positive pole is so small as compared with that at the negative pole that the heat input to the workpiece can decrease. Accordingly, when a time ratio between the EP arcing interval and the EN arcing period is controlled, the magnitude of the heat input to the workpiece can be adjusted as desired. In the case of the thin plate to be welded, an excessively large heat input to the workpiece tends to result in a burn-through and an excessively small heat input to the workpiece tends to result in an incomplete penetration, whichever is a weld defect. Accordingly, the AC pulsed arc welding process in which the heat input to the workpiece can be controlled is indeed suited for welding thin plates.
FIG. 1
is a chart illustrative of changes in welding current and voltage Iw and Vw with time, which are shown in timed relation with each other. The AC pulsed arc welding process will now be discussed with reference to the charts shown in FIG.
1
.
(1) Timings t
1
to t
2
: (Peak Current Supply Time Tp)
As shown by the waveform Iw, a peak current Ip is supplied during the peak current supply time Tp with the welding wire set to a positive pole. In general, the peak current supply time Tp and the peak current Ip are preset to respective values sufficient to allow a single molten droplet of the welding wire, then melted as a result of arc heating, to transfer for each pulse. Also as shown by the waveform Vw, during the peak current supply time Tp, a peak voltage Vp corresponding to the peak current Ip is supplied between the welding wire and the workpiece to be welded that are then held positive and negative, respectively.
(2) Timings t
2
to t
3
: (EN Current Supply Time Ten)
At the timing t
2
, the EP polarity (i.e., the polarity of the welding wire that is held positive) is switched over to the EN polarity (i.e., the polarity of the welding wire that is held negative) and, as shown by the waveform Iw, an EN current Ien is supplied during the EN current supply time Ten. In general, the EN current supply time Ten and the EN current Ien are preset to respective suitable values determined in dependence on the type of material for the workpiece to be welded, the plate thickness, the shape thereof and others.
Also, as shown by the waveform Vw, an arc generated during a duration of the EP polarity is extincted at the timing t
2
and, in order for the arc to be re-ignited instantaneously during the subsequent duration of the EN polarity, it is necessary for a high re-igniting voltage Vrs of about 200 to about 500 volts to be applied at the time of the polarity switching. In the event that the re-igniting voltage Vrs is not applied or is too low at the time of the polarity switching, the arc will no longer be re-ignited during the duration of the EN polarity and an arc interruption may therefore result in. On the other hand, if the arc is re-ignited during the duration of the EN polarity as a result of application of the re-igniting voltage Vrs at the timing t
2
, an EN voltage Ven corresponding to the EN current Ien is applied during the EN current supply time Ten between the welding wire and the workpiece to be welded that are then held negative and positive, respectively.
(3) Timings t
3
to t
4
: (Base Current Supply Time Tb)
At the timing t
3
, the EN polarity is again resumed from the EP polarity and, as shown by the waveform Iw, a base current Ib is supplied during the base current supply time Th. This base current supply time Tb is determined by a base current supply time control (hereinafter referred to as a “Tb control”) as will be described later. Specifically, the base current supply time Tb is so determined that the sum of an integral of the difference between the peak voltage Vp and a preset voltage Vs during the peak current supply time Tp, that is, ∫(Vp−Vs)dt, and an integral of the difference between the base voltage Vb and the preset voltage Vs during the base current supply time Tb, that is, ∫(Vp−Vs)dt, will be substantially zero, as shown by the following equation.
∫(
Vp−Vs
)
dt
∫(
Vb−Vs
)
dt=
0
(1)
Also, as shown by the waveform Vw, the arc generated during a duration of the EN polarity is extincted at the timing t
3
and, in order for the arc to be re-ignited instantaneously during the subsequent duration of the EP polarity, it is necessary for a high re-igniting voltage Vrs to be applied as is the case with that occurring during the EN current supply time Ten. If the arc is re-ignited during the duration of the EP polarity, a base voltage Vb corresponding to the base current Ib is applied during the base current supply time Tb between the welding wire and the workpiece to be welded that are then held positive and negative, respectively.
Subsequent to the timing t
4
, the peak current supply time Tp, the EN current supply time Ten and the base current supply time Tb are sequentially repeated to perform the arc welding.
FIG. 2
illustrates a block diagram of the prior art welding power supply apparatus used in the practice of the above discussed AC pulsed arc welding process. Referring now to
FIG. 2
, the prior art welding power supply apparatus shown therein is electrically powered by a commercial mains AC, predominantly three-phase 200/220 volts.
The illustrated welding power supply apparatus includes an output control circuit INV that is made up of a primary rectifier circuit for rectifying an alternating current power from the commercial mains AC, a smoothing circuit for smoothing the rectified voltage having ripples, an inverter circuit for converting the smoothed direct current voltag
Mondori Hiroyasu
Shiozaki Hideo
Takahashi Norihito
Tong Honjyun
Ueyama Tomoyuki
Daihen Corporation
Scully Scott Murphy & Presser
Shaw Clifford C.
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